1. Field of the Invention
The present invention relates to electronic accessories and, more specifically, to accessaries for battery powered electronic devices.
2. Description of the Prior Art
Dumb accessories for cellular telephones typically attach to a telephone, take an audio signal from the telephone and use the signal to provide an added feature. For example, a clip-on speaker phone accessory uses an amplifier to transform a cellular telephone into a speaker phone. A dumb accessory does not receive feedback from the telephone and, therefore, it must be designed to handle the worst case characteristics of the telephone.
With a clip-on speaker phone, the gain and power available are limited by the minimum voltage specification of the battery voltage range in the telephone. Typically, total harmonic distortion (THD) of less than 5% is desirable in an audio speaker device. However, voltage in a lithium battery may vary (depending on the charge state of the battery) between 4.2V to 3V. To maintain THD at the desired level, most engineers choose a low amplifier gain for the speaker phone so as to avoid clipping when the battery discharges and as the battery voltage approaches 3V, rather than choosing a high amplifier gain that would permit maximum power output when the battery is fully charged. Thus, while the amplifier prevents clipping at low battery voltage, it is incapable of realizing maximum power output when the battery is fully charged.
Therefore, there is a need for a circuit that adjusts amplifier gain as a function of battery voltage in a dumb accessory.